1. Field of the Invention
The present invention relates to signal detecting method and apparatus for processing a reproduction signal from an optical disc in which additional information such as an address or the like is recorded by wobbling a groove for tracking and, more particularly, to a signal detecting method and apparatus which can accurately detect Additional information from the optical disc.
2. Description of the Prior Art
A CD-R disc is known as a writable optical disc. In the CD-R disc, additional information such as an address or the like is recorded by wobbling a groove in advance. That is, for example, a carrier of a predetermined frequency (for example, 22.05 kHz) is modulated by additional information such as an address or the like. On the basis of the additional information modulated, the groove for tracking is wobbled.
The frequency of the signal which was wobble recorded as mentioned above is sufficiently higher than that of a tracking error signal. Therefore, by separating a predetermined frequency component from the tracking error signal, a wobble signal can be detected.
That is, for instance, a groove 51 is provided along a track on a writable optical disc such as a CD-R or the like as shown in FIG. 3. The groove 51 is wobbled on the basis of the additional information modulated. A deviation of the wobble is set to a small value of about 30 nm so as not to exert an influence on a signal component by a pit.
A two-divided detector comprising detectors 61a and 61b as shown in FIG. 4 is provided for such an optical disc. When a laser beam 53 is irradiated to the optical disc, the reflection light is supplied to the two divided detectors 61a and 61b. By performing a subtraction between outputs of the two divided detectors 61a and 61b, a tracking error signal is obtained. On the basis of the tracking error signal obtained as mentioned above, a tracking servo is performed.
Although the groove 51 has been wobbled, since a frequency of the wobble is enough. higher as compared with a frequency of the tracking error signal, the laser beam 53 doesn't trace along the groove wobbled but traces the center of the track as a whole.
When the laser beam 53 is allowed to trace along the track, a signal as shown in FIG. 5A is obtained from the detector 61a and a signal as shown in FIG. 5B is obtained from the detector 61b. When a subtraction is performed between the outputs of the detectors 61a and 61b and a tracking error signal component is eliminated from the subtracted value, a wobble signal as shown in FIG. 5C is detected. By demodulating the wobble signal, the additional information such as an address or the like can be obtained.
U.S. Pat. No. 4942565 discloses an optical disc apparatus using wobbling of tracks on which control information is recorded. Further, U.S. Pat. No. 5,185,732 discloses an optical disc apparatus in which information is recorded on tracks of an optical disc. Furthermore, U.S. patent application Ser. No. 095288 filed on Jul. 21, 1993 of the present applicant discloses an apparatus reproducing information recorded in a pre-groove.
When a signal is recorded in such an optical disc, pits 52a and 52b are arranged in the wobbled groove 51 wobbled. Conventionally, there is a problem such that, when such pits 52a and 52b are provided, in reproducing, signals of the pits 52a and 52b exert an influence on the wobble signal.
That is, inherently, the signal components of the pits ought to be cancelled by performing a subtraction between the outputs of the two detectors 61a and 61b. In order to raise a sensitivity of a focusing servo, however, a spot diameter of a laser beam is ordinarily converged to a small diameter of about 100 .mu.m. When the spot diameter of the laser beam is converged as mentioned above, there is a case where a position of the laser beam for the detector is moved due to the temperature characteristics or the like. In this case, a difference of the signal levels of the two detectors 61a and 61b occurs, and as shown in FIG. 5C, a differential signal between the detectors 61a and 61b doesn't become a simple wobble component but includes influence portions 71a, 71b, and 71c influenced by the pits as shown in the hatched portions. When the signal components due to the influence by the pits remain, a problem occurs such that the detection of the wobble signal as a microdeviation becomes unstable. Although such an influence of the signal component is improved to a certain extent by a non-linear operation such as a peak holding or the like, since the pit is influenced by the optical diameter, it cannot be completely improved.